The present invention relates to a diagnostic substance which contains at least one complex of lipophilic anions and metal ions, as well as the use of the diagnostic substance for investigating metabolic processes in the brain and/or central nervous system (CNS).
Changes in the activity and metabolism of neurons and glial cells are accompanied by changes in the rate of uptake and the intracellular and extracellular concentrations of numerous cations (e.g., Na+, K+, Ca++, Mg++, Zn++). In pathological processes, particularly ischemia, tumors, inflammations and neurodegenerative disorders (dementias, Alzheimer's disease), there is altered neuronal and glial cell activity and shifts in cation equilibria occur. In addition and particularly in the case of degenerative changes, tissue components have an altered cation binding behavior.
Changes in cation metabolism in the CNS could previously not be investigated in routine diagnosis.
Attempts to investigate cation metabolism by means of nuclear resonance spectroscopy are known, wherein a prerequisite is that measurable isotopes are present. This condition is in fact fulfilled in the case of the measurement of potassium ions, but the three-dimensional resolution obtained in these investigations is usually poorer than in the case of isotope investigations by means of a gamma camera. And in the case of other isotopes, such as calcium, for example, an investigation is generally not possible by means of NMR spectroscopy.
Paramagnetic manganese (Mn++) can only be used as a tracer for calcium metabolism in animal experiments—after opening the blood-brain barrier. A transfer of this method to humans, however, has previously not been possible, since the channeling of non-toxic quantities of manganese through the blood-brain barrier and utilizing imaging for magnetic resonance in humans has not as yet been successful.
The single method that could be used up to now for the direct measurement of metabolic changes in the CNS is the positron emission tomographic measurement of glucose metabolism (18fluorine-deoxyglucose PET).
Due to the high cost of the equipment, however, this method is utilized only at a few selected clinics and it can be transferred only with difficulty to the arena of the physician's practice.
Today, the changes in ion metabolism are usually indirectly investigated, for example, via changes in the mobility of water molecules in magnetic resonance tomography (MRT) or via cerebral blood flow measurements.
Complexing agents which complex the isotopes of specific heavy-metal ions are utilized, among others, for the so-called tracer technique. The heavy-metal ion bound in these complexes only plays the role of a “reporter”, which will indicate where the complexed compound diffuses in the body. The actual diagnostic substance is the complexed compound.
It is a disadvantage in this indirect measurement, however, that the consequential changes that follow disrupted metabolism—such as altered blood flow or an altered resonance behavior that can be measured by means of magnetic resonance—are measured exclusively. And as yet it can only be estimated as to how and under what conditions disruptions in cellular metabolism lead to changes in blood flow or alterations of water proton resonance.
A direct measurement of cation metabolism, in contrast, has the advantage that a direct view into cellular metabolism is offered. In therapy monitoring, the direct determination of cation metabolism could be even more important than for simply establishing a diagnosis of a CNS disorder, since it is still not clear how the recovery of metabolism affects the above-named consequential changes (blood flow, water proton resonance). For this reason, it is completely conceivable that cellular metabolism recovers without a change in the indirect water signals that can be measured with magnetic resonance or that changes occur before recovery.
In the case of the above-mentioned indirect method for investigating ion metabolism, the selection of a suitable tracer is of the utmost importance.
Here, one of the most important criteria in the selection of the complexing agent and the metal ion to be complexed in this method is the stability of the complexed compound in a physiological environment. This is because only an intact tracer in which the metal ion is still bound to the complexing agent, makes possible the above-described detection of the complexing agent with methods in which metal ions are detected.
In this context, the following are known, for example: the use of the non-radioactive isotopes of thallium and of the gamma radiator 201Tl, the use of the non-radioactive isotopes of cobalt, and of the gamma radiator 57Co and the positron emitter 55Co, the non-radioactive isotopes of manganese and the positron emitter 52 mMn, as well as the non-radioactive isotopes of lead, iron and nickel.
The tracer, and to be more precise, the reporter in the tracer, is then detected for paramagnetic isotopes (manganese, cobalt, iron) by means of nuclear magnetic resonance methods, [or] positron emission tomography, PET, for the detection of positron emitters and single-photon emission tomography, SPET, for the detection of gamma radiators.
This stability of the tracer molecules, which brings about the circumstance that almost no free metal ions are retained in the investigated tissue, nevertheless requires that imaging detection methods are sufficiently rapid in order to produce an image in the period of time in which the tracer is found in the region of investigation. In a study by Ballinger et al., Appl. Radiat. Isotop. Vol. 38, No. 8, pages 665-668, 1987, this problem was discussed precisely in connection with SPECT investigations with a gamma camera in the measurement of cerebral blood flow. Since the gamma camera at that point in time required 20 to 40 minutes in order to accumulate an image, the suitability of two tracer molecules was discussed for this method. A comparison was made between two lipophilic complexes, i.e., technetium-99m-diethyldithiocarbamate (99mTc-DDC) und thallium-201 diethyldithiocarbamate (201Tl-DDC). Both substances were investigated as to whether they were suitable for the imaging method utilized for blood flow measurements in the brain.
In this way it was established that both complexed compounds show a good cerebral uptake due to their lipophilic nature, but have very different retentions. This difference in retention was explained by the fact that 201Tl-DDC decomposes spontaneously in the brain and ionic 201Tl is formed, which cannot cross the blood-brain barrier. In contrast, 99 mTc-DDC has an essentially lower rate of decomposition in the brain, for which reason, the compound is retained there to a lesser extent.
In spite of this knowledge, it was estimated that 201Tl-DDC was less suitable in the described method among others, since it is not optimally suitable for the SPECT method due to its gamma emission. In addition, it was established that 201thallium is disadvantageous due to its half-life of three days.
No instance has previously been known, however, in which the decomposition of a metal chelate complex in a physiological environment has been utilized in a targeted manner for diagnosis. It is also not known that metal chelate complexes have been selected or synthesized according to this criterion.
The great diagnostic potential has not been recognized that this decomposition of lipophilic heavy-metal complexes in crossing the blood-brain barrier and the retention associated with it opens up a way for the use of these complexes for investigating ion metabolism in the CNS.
All documented efforts exclusively bear on suppressing the decomposition of the utilized tracer as much as possible in a physiological environment or at least delaying it.
And up to today, this knowledge has not been utilized to develop a method that makes possible a direct measurement of the alteration of cation metabolism in vivo.
In a study published in 2004 by Goldschmidt et al., Neuroimage 23(2):638-47, the use of thallium acetate in a high-resolution, non-radioactive method was [described]. This method was primarily conducted in order to make possible a histochemical, high-resolution representation of neuronal activity. The principle for this is that neuronal activity and potassium (thallium) uptake are closely coupled, as is known, and the thallium compound served as a tracer for potassium ions. The basis of the described method is so-called autometallography, which involves a standard detection method for heavy metals. The application of this method is explained in an animal model in which autometallography has been performed as a histochemical method after the tissue has been removed.
The fact that very large amounts of thallium acetate had to be utilized in order to correspond to the sensitivity of the method is a disadvantage with this method. These high thallium doses and the fact that the detection of thallium is made histochemically after removal of tissue make the use of the method impossible in humans. The dose would be deadly even for experimental animals, but the experimental animals were sacrificed as early as 15 minutes after the administration of thallium acetate, and the brains were removed for histochemical investigation.
Apart from the impossibility of working with this method on humans, the use of water-soluble thallium salts still has the disadvantage that the regional thallium distribution is also determined by regional differences in the potassium conductivity of the blood-brain barrier. This limits its use, particularly in the analysis of pathological changes in which the blood-brain barrier is also altered, and also makes it difficult to compare the cellular thallium uptake in different brain regions.